1. Field of the Invention
This invention relates to a stacking apparatus for arranging sheet of paper in the form of a stack, and more particularly, to a sheet paper stacking apparatus which receives sheet of paper one by one and arranges them in a stack with the front edge and one side edge of each sheet aligned with a front reference line and a side reference line, respectively. The present invention is particularly advantageous to be employed in duplex copying or printing machines which make reproductions or prints on both sides of the paper.
2. Description of the Prior Art
When making reproductions or prints on both sides of a sheet of paper by a duplex copying or printing machine, a common practice is that a reproduction or print is first formed on one side of the sheet of paper, and then the sheet of paper is turned upside down for processing the remaining side thereof. An electrophotographic copying machine usually comprises a photosensitive drum which is journaled in a frame to rotate in a predetermined direction at a constant speed; on the other hand, a printing machine has a printing roller which is rotatably supported in a frame. A sheet of paper is brought into contact with the rotating photosensitive drum or the printing roller for transferring the developed image on the drum or the ink image on the roller onto the surface of the sheet of paper. It is therefore required that the sheet of paper be transported in synchronism with the rotation of the drum or roller and with a particular spatial relationship with respect to the drum or roller in order that the image can be properly transferred onto the sheet of paper.
In duplex reproducing and printing operations, the above-noted problem of timing and alignment is more severe as compared with the ordinary single side reproducing and printing operations. Because the sheet of paper, upon completing reproduction or printing on one side, must be turned upside down, and again it must be brought into contact with the photosensitive drum or the printing roller for the second reproducing or printing operation. Thus, a tray is usually provided to receive sheets of paper after completing reproduction or printing on one side so as to facilitate the required timing and alignment procedure. The use of such a tray is advantageous in a duplex processing, but there are several requirements to be met. For example, when sheets of papers are introduced into such a tray one by one after processing of the front side, they must be stacked one upon another with their front edge and one side edge properly aligned with front and side reference lines, respectively, ready for the reverse side processing.
Conventionally, use has been made of an end block and side blocks, respectively provided at the front end and on both sides of the tray. With such a conventional structure, sheets of papers are introduced into the tray by a transporting mechanism and they are left there to themselves to take the proper position in abutment against the blocks. This structure is disadvantageous because the side blocks, and possibly the end block too, must be made adjustable and their locations must be individually determined in accordance with the size of the sheets of papers used. Moreover, if a sheet of paper is somewhat disoriented, i.e., oriented aslant with respect to the centerline of the normal sheet paper travelling path, or shifted sideways when introduced into the tray, the front edge of the sheet paper could bump into the end of one side block, resulting in the occurrence of jamming.
It has also been proposed to use a pressing plate for the purpose of attaining a perfect alignment of the sheets of paper when stacked in the tray. However, such a pressing plate must be operated in association with the transport of the sheets of paper to bring them to properly abut against the reference walls. In addition, the stroke of the pressing plate must be adjusted in accordance with the size of sheets of paper used. Therefore, use of such a pressing plate necessarily complicates the structure of a sheet paper stacking apparatus and, therefore, it is rather disadvantageous in a practical sense, though sheet papers can be properly aligned with reference lines.